Re: Good & Bad Maxwell Caps, MMC questions?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Bill,
         Here's my opinion on MMC's as a keen user FWIW:

On 16 Jul 01, at 11:16, Tesla list wrote:

> Original poster: "Bill Vanyo by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<vanyo-at-echoes-dot-net>
> 
> I just blew my Maxwell cap (0.06uF, 30kV, catalog #37321 - type with
> terminals on top).
> 
> I was never sure whether this was a good cap for Tesla use.  I've seen
> guidelines in the past that said those with the terminals on top
> (instead of on opposite sides) generally didn't have a long life.  It
> worked great for a while, but now it's dead (cap makes a sort of
> fizzling sound when I first power up).
> 
> Can anyone confirm that this is indeed not a good cap to use, or is this
> what I could expect with any Maxwell cap?  And if mine was bad, which
> are good?  The reason I'd like to get another Maxwell (if I can find a
> more reliable one) is that it's "plug-and-play" - no assembly required,
> and compact.
> 
> Are MMC's really any better?  To what extent are they "self healing"? 
> To build a really sturdy MMC with the geek group caps (942c), for use
> with 15/180 NST (three 15/60's), what is the minimum string length and
> number of strings to handle the voltage and current they'll see?  I can
> adjust upwards to get my desired capacitance, which is between 0.06uF
> and 0.045uF --- 0.06 because that's what I was using and it worked,
> 0.045 because that's the "right" LTR value for 15/180 with a static gap,
> but I don't know (I haven't calculated yet) if my primary has enough
> extra turns that I'd still be able to tap it in tune with 0.045 (I
> currently tap around 7.5 with my biggest topload, and have 10 turns).  I
> understand that the strings need to be of adequate length to handle the
> voltage (though I'm not sure what that length is to get a long MMC life
> with 15kV AC in tesla coil use).  Am I right that there is also some
> minimum number of strings needed for adequate current handling (180ma,
> in my case)? 
> 
> BTW (and I'm not sure, but I think I've heard this reported before), my
> coil seemed to be performing better than it ever had shortly before it
> died.
> 
> 	Thanks,
> 	-  Bill Vanyo

I don't use self-healing types as I'd rather know if I botched the 
design. With caps generally I think you get what you pay for. A 
robust MMC is pretty expensive to build. Something I'm doing must be 
right if I haven't blown one yet. I have one running in a coil 
pulling around 600A+ peak primary current. There are 15 strings in 
parallel for that model. It is designed for moderate use with rest 
periods although it got a thrashing at our open day last year. If I 
wanted to switch that coil on and let it run until the gap widened 
beyond firing point with wear I would double the number of strings 
(and of course have to quadruple the number of caps to get the same 
C). That would push the cost from around $200 to $800 for the caps 
alone. Incidentally, $800 NZ is about what it cost me to land a CP 
25nF 20kVACW capacitor. With the benefit of hindsight, I know where I 
should have spent that money (and no risk of an oil explosion either 
;) 

Regards,
Malcolm